Hilmir Enters The Wood

The way the invisibility charm works, you remain invisible for the duration of that charge, even if attacking or taking any other action. The other Robes (five or so) are focusing the fire at the moment, but Jarret and the wary one are heading in your general direction. You're about 30ft from the entrance to the Ruins, and these two Robes are about 30ft from you (think of it like a triangle).

If any of that isn't clear feel free to ask clarifying questions.

Let's do it like this, while Invisible an accompanying DEX save means any action is done quietly and the guards are unaware. So Dreli and Korban going into the temple would trigger a DEX save, if it's a fail someone hears something going in, if it's a pass the Robes are unaware.

As for combat, the weapon die is rolled and the result is taken away from the target's armor and other stats to determine damage. The advantage of Invisibility means that any attacks are upgraded to a 1d12 damage die. Also, while invisible you can only be targeted by an opponent's attack if DEX save is failed in the previous round (meaning they have a good idea approximately where you are). Any attack against an invisible opponent would require a successful STR save or miss wildly.

If the intention is for Hilmir to try and perform sort of a sneak-up coup-de-grace move, this would require both a DEX save and STR save. A success means he goes down clean, a fail on either means it's just a normal attack and 1d12 damage die would be rolled.

Also as a note, while the wary guard has a short sword, you've seen other Robes with spellbooks so keep in mind that a couple of these ones may have spells as well.

Korban uses a short spear generally, and when Hilmir first encountered the frogmen Korban had a spear and the others wielded large rocks. However, the guards at the frogmen village had short swords.